1. Field of the Invention
The present invention relates to an image forming apparatus for forming color images based on electrostatic copying processes such as copiers, facsimiles, and printers. The present invention further relates to a toner used for the color image forming, an image forming apparatus using the toner, and an image forming method thereof.
2. Description of the Related Art
In image forming based on an electrophotographic process, a latent image is formed by means of electrostatic charge on an image bearing member having a photosensitive layer which comprises photoconductive substances and the like, charged toner particles are adhered on the latent electrostatic image to form a visible image, and then the visible image is transferred onto a recording medium such as paper and fixed on the recording medium to be an output image. In recent years, there have been rapid developments from monochrome image technologies toward full color image technologies of copiers and printers using electrophotographic processes, and the market of full color image technologies increasingly tends to expand. Typically, in color image forming based on a full color electrophotographic process, all colors are reproduced by superimposing three color toners of yellow, magenta, and cyan which are three primary colors or four color toners with black color toner added to the three primary colors. Therefore, to obtain a full color image having excellence in color-reproductively and color vividness, the surface of the fixed toner image must be smoothed and evened to some extent to reduce scattering of light. For this reason, there were so many conventional types of full color copiers or the like which have a middle level of image glossiness to high level image glossiness of 10% to 50%.
In color image forming based on an image developing method using a two-component developer, when the developer is stirred, toner particles are fixed and flocculated each other by compression force worked among carriers. In color image forming based on an image developing method using a one-component developer, toner particles are flocculated each other by pressure, frictional force or the like when the toner is made into a thin layer on a developing roller. In both two-component developing method and one-component developing method, a toner is semi-molten to cause toner-fixed aggregate by heat generated from friction of axes such as mixing fans and screws when mixing the developer. The toner-fixed aggregate is developed on or attached to an image to appear as thick and not-small spots on the image. When the image is transferred onto a paper sheet, the toner-fixed aggregate serves as a spacer between the paper sheet and a photoconductor, resulting in a loss of color of the image at that portion into white color. Particularly in color images, abnormal images easily stand out when comparing with monochrome images, and high resolution images having fine-textured tones and fine color reproductivity are required, and therefore abnormal images brought about by such a toner-fixed aggregate has become an issue. In particular, quality of color images is substantially affected by magenta colorants from the viewpoint of the relative luminous efficiency of humans.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2004-77664 discloses a magenta toner for developing electrostatic images which comprises a colorant in which the colorant is a predetermined compound, and the toner is produced by dissolving a toner composition containing a modified polyester resin capable of a urea-binding in an organic solvent, subjecting the toner composition to a polyaddition reaction in an aqueous medium, and rinsing the dispersion liquid to remove the solvent from the dispersion liquid. In addition, Japanese Patent Application Laid-Open (JP-A) No. 2003-215847 discloses a magenta toner for electrophotography which comprises a binder resin and a colorant, in which the colorant comprises a naphthol pigment having a predetermined structure, the shape factor SF-1 of the toner is 110 to 140, and the volume average particle diameter of the toner is 2 μm to 9 μm. However, there is no disclosure in the invention on improvements in color reproduction in red color region through the use of the combination of specific naphthol pigments and a specific yellow pigment.
As for a method for fixing a toner image on a recording medium, the following image fixing method is often used, in which an image fixing roller or an image fixing belt having a smooth surface is heated and pressed firmly to a toner to thereby fix a toner image. This method has advantages of having high thermal conductivity and enabling high-speed fixing and imparting gloss and transparency to color toners, while it causes so-called offset phenomenon in which part of a toner image adheres to the surface of a fixing roller and spreads to other images, because a surface of a heating and fixing member is made contact with a molten toner under pressures and then they are isolated from each other. With a view to preventing the offset phenomenon, the following method is typically employed, in which a surface of a fixing roller is formed with silicone rubber and fluororesin each having excellent releasing property, and a releasing oil such as silicone oil is further coated on the surface of the fixing roller. This method is fairly effective in terms of preventing offset phenomenon of toners, however, it requires a device for supplying a releasing oil, and a large-sized image fixing unit must be prepared, resulting in high cost. Therefore, for monochrome toners, the following method tends to be widely used, in which viscoelasticy of a fused toner is enhanced so that the fused toner particles are not is broken internally by controlling the distribution of molecular mass of a binder resin, and no releasing oil is coated on a surface of a fixing roller or only a minute amount of releasing oil is used and coated thereon by adding a releasing agent such as wax in the toner.
However, in color toners, viscoelasticy of a molten toner must be lowered, because it is necessary to smooth a surface of a fixed image to improve color reproductivity. Color toners are more likely to cause offset phenomena than in monochrome toners which have no glossiness, and it is much more difficult to use an oilless toner in an image fixing unit and to use a minute amount of a releasing oil to coat a surface of a fixing roller. In addition, when a releasing agent is included in a toner, adhesive strength of toner increases and transferring properties of toner against a transferring sheet degrades, causing a problem that interior part of an image forming apparatus is smeared because the releasing agent in the toner contaminates frictional electrification members such as carriers, and charge properties of the toner degrades.